A PTC device is used as circuit-protection device which protects, for example, electric circuits, in a variety of electric or electronic apparatuses. Such PTC device shows an electric resistance which changes depending on a temperature. In general, the PTC device has such property that its resistance rapidly increases when its temperature elevates from a room temperature so as to exceed a specific threshold temperature called a trip temperature. The property as above, namely, increase, preferably rapid increase in the resistance in association with increase in temperature, is called a “PTC characteristic”, and such a rapid increase in resistance is called “trip”. When concentrated attentions are paid to a switching function of a PTC device as will be described later, a trip temperature is also called a switching temperature.
As described above, the PTC device is used by being integrated into an electric circuit of an electric or electronic apparatus. For example, when an excess of current passes through the electric circuit including the PTC device for some reasons while such an apparatus being used so that the temperature of the PTC device accordingly elevates to the threshold temperature, or otherwise, when an ambient temperature around the apparatus rises to elevate the temperature of the PTC device to the threshold temperature, the resistance of the PTC device rapidly becomes higher, namely, the PTC device trips. Particularly when a PTC device is used as a protective circuit in an electronic apparatus, it is essential that the resistance change of the PTC device from a temperature just below the threshold temperature to a temperature just above the threshold temperature should be rapidly large and such change should be at least 100 times, preferably 1,000 or more times larger. Especially, a function of the PTC device showing such a rapidly large change is called a “switching function”.
In an actual temperature-resistance curve obtained from a PTC device, the resistance change of the PTC device from the temperature just below the threshold temperature to the temperature just above the threshold temperature is an steep change within a certain temperature range, but not a stepwise change (that is, a change showing a curve slope of substantially 90°). Accordingly, the wording of “a change in resistance from the temperature just below the threshold temperature to the temperatures just above the threshold temperature” herein used throughout the present description is intended to mean a ratio of a resistance found just after such a rapid change to a resistance found just before the rapid change. In general, the PTC device shows a very large change in its resistance, and therefore, the resistance found just before such a rapid change may be regarded as being equal to a resistance found at a room temperature in view of practical use.
For example, referring to the measured data indicated in FIG. 2, a device of Example 1 showed a rapid increase in its resistance within a temperature range between about 100° C. and about 130° C. In this case, the change in resistance corresponds to a ratio of a resistance at 130° C. to a resistance at 20° C., and this ratio of the change in resistance is in the range of between about 104 and about 105.
When such a PTC device is incorporated into an electric circuit to be disposed in a power supply line, the PTC device of which resistance has increased substantially shuts off a current (namely switches off) so as to thereby prevent a possible failure of the apparatus beforehand. When such a PTC device forms a protection circuit in an apparatus in another embodiment, the PTC device becomes of a higher resistance because of an abnormal rise of an ambient temperature, and consequently, the PTC device switches to stop the application of voltage in the protection circuit so as to prevent a failure of the apparatus beforehand. This “switching function” of the PTC device is well-known to the art, and various kinds of the PTC devices have been used. For example, a PTC device having such “a switching function” is incorporated into a protection circuit in an electric circuit of a secondary battery for a cellular telephone. When an excess of current passes through the secondary battery which is being charged or discharged, the PTC device shuts off the current to protect the cellular telephone, for example, the secondary battery thereof.
The trip or switching temperature and the switching function as mentioned above are also disclosed, for example, in Patent References 1 and 2 described below. These References can be referred to in relation to the present invention, and the contents disclosed in these References constitute a part of the present description by reference.
As one of the conventional PTC devices, there is known a polymer PTC device which comprises a layered (or planar) polymer PTC element made of a thermoplastic crystalline polymer material as a base material which contains a conductive filler dispersed therein as electrically conductive particles (see for example Patent References 3). The layered polymer PTC element can be manufactured by extruding a high density polyethylene which contains an electrically conductive filler such as carbon black dispersed therein. A polymer PTC device is fabricated by disposing suitable electrodes on both main surfaces of the polymer PTC element. For example, metal foil electrodes are used as such electrodes. The metal foil electrodes are bonded on the layered polymer PTC element, for example, by thermo-compression bonding.
Why the polymer PTC device can exhibit the above-described switching function can be explained as follows with reference to FIGS. 1(a) and 1(b): FIGS. 1(a) and 1(b) schematically show electrically conductive particles (e.g. carbon black powder) which are dispersed in a thermoplastic crystalline polymer of the polymer PTC element, illustrating the dispersing conditions of the conductive particles which are found before the trip (at a normal or room temperature or under normal conditions) and upon the trip, respectively. The thermoplastic crystalline polymer includes a crystal portion in which the polymer chains are regularly and densely aligned, and an amorphous portion in which the polymer chains are present coarsely and randomly. Consequently, it is physically hard for the conductive particles to enter the crystal portion having the polymer chains densely aligned therein, and thus, the conductive particles are concentrated and collected in the amorphous portion of the polymer. This fact means that the conductive particles are densely present in contact with one another in the amorphous portion of the polymer, and it is considered from this phenomenon that the polymer PTC element is low in its electrical resistance.
On the other hand, when the temperature of the polymer PTC element rises, the crystal portions in which the polymer chains have been regularly and densely aligned at a normal temperature gradually transfer to an amorphous state where the polymer chains are present at random, because the molecular motions become more active with an increase in temperature. When the temperature of the polymer PTC element reaches the trip temperature which is around a melting point of the crystalline polymer, the crystal portions of the crystalline polymer start melting, so that the amorphous portions of the polymer increase. This state of the PTC element is schematically shown in FIG. 1(b). In this state, the movement of the conductive particles, which has been restricted due to the crystal state at a normal temperature, becomes possible. As a result, appreciable amounts of the conductive particles are away from one another, and thus, it is considered that the electric resistance of the polymer PTC element becomes higher.
The above increase in the electric resistance of the polymer PTC element can be achieved by making use of a phenomenon of conductive particles' moving away from one another due to the volume expansion of the polymer in addition to or instead of the melting of the crystal portions. However, to achieve a larger change ratio in electric resistance (i.e. a ratio of a resistance upon a trip/a resistance found before the trip (or a resistance found at a normal temperature), it is preferable to use, for the polymer PTC element, a polymer of which crystal state becomes amorphous in place of and preferably in addition to exhibiting the volume expansion. When a non-crystalline polymer such as a thermosetting resin is used to manufacture a PTC element, it is possible to achieve a slight change (usually several times to several tens times larger) in electrical resistance attributed to a transition point such as a glass transition point, but it is impossible to achieve a change ratio in resistance (generally at least 1,000 times larger) which makes it possible to exhibit a switching function required to be used as a circuit protection device.
In order to improve the characteristics of the above mentioned polymer PTC elements, various new studies have been continuously carried out: for example, there has been carried out a study to obtain a large change in resistance and an acute rise in a temperature-resistance curve while lessening an initial resistance of a PTC device at a room temperature. As one of such examples, a study is reported wherein nickel powder is used as an electrically conductive filler (see for example Patent References 3).    Patent References 1: JP-B-4-28743 (1992)    Patent References 2: JP-A-2001-85202 (2001)    Patent References 3: JP-A-5-47503 (1993)